This invention relates to a lithographic printing plate material which utilizes the silver complex diffusion transfer process.
The lithographic printing plate consists of greasy ink receptive oleophilic image portions and ink repellent oleophobic non-image portions, the latter being generally water receptive hydrophilic areas. Therefore, the customary lithographic printing is carried out by feeding both water and colored ink to the printing plate surface to allow the image portions to receive preferentially the colored ink and the non-image portions preferentially water and then transferring the ink deposited on image portions onto a substrate such as paper. In order to obtain a print of good quality, therefore, it is necessary that oleophilicity of the image portions and hydrophilicity of the non-image portions are both strong enough so that when water and ink are applied, the image portions may receive sufficient amount of ink while the non-image portions may completely repel the ink.
In the case of lithographic printing plate material which utilizes the silver complex diffusion transfer process for making the printing plate, especially the material having a nuclei layer on an emulsion layer, the silver salt grains where latent images have been formed by exposure bring about chemical development by development to change into black silver to form hydrophilic non-image portions and on the other hand, unexposed silver salt grains become silver salt complexes by the action of silver halide complexing agent contained in developer and diffuses into the surface nuclei layer, where the diffused silver salt complexes undergo physical development due to the presence of the nuclei to form image portions mainly composed of ink receptive silver.
In thus obtained lithographic printing plate, the silver layer precipitated on gelatin is utilized as ink receptive image portions and so resistance of the image portions against mechanical wearing is not sufficient as compared with the general lithographic printing plates such as PS plate and the like, resulting in the defects such as wearing-off of image portions and gradual decrease of ink-receptivity of image portions.
When hardness of gelatin is increased or amount of the physical development nuclei is increased in order to remove these defects, there occurs scumming to greatly reduce the printing endurance of lithographic printing plate. Generally, mechanical strength of image portions and scumming interfere with each other to give limitation to printing endurance of the lithographic printing plate and new technique to overcome this limitation has been much demanded.
As a result of the inventors' intensive researches in an attempt to improve printing endurance of the lithographic printing plate from the point of balancing between physical development and chemical development, it has surprisingly been found that the mechanical strength of image portions can be markedly increased without increase of scumming by containing a benzotriazole or its derivatives in a constitutional layer of the lithographic printing plate material and this invention has been accomplished.